A piezoelectric device and a package

Abstract: A piezoelectric device includes a piezoelectric plate (10) provided with at least one electrode at almost a central portion of each face (106,107); a first plate (102) made of an ionic material having a recessed portion to cover the at least one electrode at one face to face the piezoelectric plate; and a second plate (103) made of an ionic material having a recessed portion to cover the at least one electrode at one face to face the piezoelectric plate. Each of the at least one electrode is packaged by direct bonding the piezoelectric plate with the first plate and the second plate so as to be sandwiched therebetween. A package includes a body member made of an ionic material provided with a recessed portion or an opening, having a flat face around the recessed portion or the opening; and a covering member made of an ionic material having a flat face to cover the body member. The flat face of the body member and the flat face of the covering member are cleaned and attached to each other so as to be direct bonded with each other using hydrogen bonds generated between the ionic materials, thereby packaging the recessed portion or the opening.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention: The present invention relates to a piezoelectric device and a surface acoustic wave filter as for example used in circuits for mobile wireless communication equipment such as portable telephones. 2. Description of the Related Art: Recently, as mobile wireless communication equipment such as portable telephones have become widely used, there has been an increasing demand for a quartz device for a reference frequency source and a filter used in such equipment. Hereinafter, a conventional ceramic package, a quartz crystal resonator, a quartz filter, and a quartz surface acoustic wave filter will be described referring to Figures 24 to 27. Figure 27 shows an exemplary structure for the conventional ceramic package. In Figure 27 , the conventional ceramic package includes a ceramic covering member 3701 and a ceramic body member 3702. On the top face of the ceramic body member 3702, a packaging glass 3702a is formed using a paste containing glass powders. A recessed portion 3702b is formed at the ceramic body member 3702 . As for the package shown in Figure 27 , a packaging operation is performed as follows: an object to be packaged is first put into the recessed portion 3702b , the ceramic covering member 3701 is overlaid thereon, and finally the container is wholly heated up to a temperature at which the glass 3702a melts. Such a packaging operation is described, for example, in Nikkei Electronics, Jan. 7, 1991 (No. 517), pp. 167. Figure 24 shows an exemplary structure for the conventional quartz crystal resonator. In Figure 24 , the conventional resonator includes a quartz plate 3401 , an electrode 3402, conductive adhesives 3403 and 3404 , supporting members 3405 and 3406 , leading members 3407 and 3408 , a supporting plate 3409 , and a cap 3410 . The electrode 3402 is formed on each face of the quartz plate 3401 . The quartz plate 3401 functions as an electric resonator, whose resonance frequencies are determined by the thickness and the elastic constant thereof. The quartz plate 3401 is connected to the supporting members 3405 and 3406 both supported by the supporting plate 3409 by means of the conductive adhesives 3403 and 3404, and thus the quartz plate 3401 is electrically connected to the supporting members 3405 and 3406 . In addition, the quartz plate 3401 is electrically connected to the outside via the leading members 3407 and 3408 via the supporting members 3405 and 3406 . Furthermore, the cap 3410 packages the quartz plate 3401 . The packaging operation is generally performed using a solder welding method. Figure 25a is a front view showing a structure for the conventional quartz filter, while Figure 25b is a back view showing the structure for the conventional quartz filter. In Figures 25a and 25b the conventional quartz filter includes a quartz plate 3501 , electrodes 3502 to 3505 , conductive adhesives 3506 to 3509 , supporting members 3510 and 3511 , leading members 3512 to 3515 , a supporting plate 3516 , and a covering member 3517 . The electrodes 3502 and 3504 are disposed so as to be opposed to the electrodes 3503 and 3505, respectively. The electrode 3502 is led to the outside by the leading member 3512 via the conductive adhesive 3506 and the supporting member 3510 . The electrode 3504 is led to the outside by the leading member 3514 via the conductive adhesive 3508 and the supporting member 3511 . Similarly, the electrode 3503 is led to the outside by the leading member 3513 via the conductive adhesive 3507 . Likewise, the electrode 3505 is led to the outside by the leading member 3515 via the conductive adhesive 3509. The electrodes 3503 and 3505 are generally earthed. Upon inputting an electrical signal to the electrode 3502 , the quartz plate 3501 resonates at frequencies determined by the thickness and the elastic constant thereof. The thus caused resonance is mechanically propagated in the quartz plate 3501 , so that an electrical field is generated between the two electrodes 3504 and 3505 , and consequently the electrical signal can be picked up from the electrode 3504 . That is, the quartz filter can function as a band-pass filter for the frequencies which are determined by the thickness and the elastic constant of the quartz plate 3501 . In addition, the quartz plate 3401 is sealed by the supporting plate 3516 and the covering member 3517 . Such a packaging operation is generally performed using the solder welding method. Figure 26 shows a structure for the conventional surface acoustic wave filter. In Figure 26 , the conventional surface acoustic wave filter includes a piezoelectric substrate 3601 , a covering member 3602, a base member 3603 , an interdigital transducer (IDT) type electrode 3604 , a bonding pad 3605, and glass 3606. As shown in Figure 26, the IDT type electrode 3604 is formed on the piezoelectric substrate 3601 . Upon inputting an electrical signal to the electrode 3604, surface acoustic waves are excited on the piezoelectric substrate 3601 , and finally only waves having certain frequencies can be propagated. Herein, an electrode pattern of the IDT type electrode 3604 and an effective piezoelectric constant of the piezoelectric substrate 3601 determine which surface acoustic waves can be propagated. According to such principles, a surface acoustic wave filter having the desired filter characteristics can be obtained by appropriately selecting the electrode pattern of the IDT type electrode 3604 , and the material for and the cutting angle of the piezoelectric substrate 3601 . The packaging is performed as follows: First, the piezoelectric substrate 3601 is fixed onto a recessed portion of the base member 3603 by using an adhesive. Thereafter, the glass 3606 is applied to the base member 3603 , and the covering member 3602 is overlaid thereon. Finally, the container is wholly heated up to a temperature at which the glass 3606 melts, thereby integrating the covering member 3602 with the base member 3603. The above-mentioned surface acoustic wave filter and package have some problems. For example, according to the conventional glass packaging method, a glass paste is used for packaging, so that gas is disadvantageously generated from the glass paste. As a result, the device characteristics are likely to deteriorate due to age. In addition, the use of glass as the adhesive necessitates the need for additional space for melted glass to flow for the packaging portion, which makes it difficult to miniaturize the device. Furthermore, such a complicated manufacturing process leads to a high production cost. Moreover, the solder welding method has a problem in that the height of the package is made higher as shown in Figures 30 and 31, which makes the face mounting of the package inappropriate. In addition, not only for the structural reason, the above-mentioned quartz crystal resonator and quartz filter cannot be readily made compact because many components such as the supporting members, leading members, supporting plates, and covering members should be included therein. Moreover, the use of the conductive adhesive for the bonding of each component generates gas, and consequently the package is likely to deteriorate due to age. SUMMARY OF THE INVENTION The piezoelectric device according to the invention comprises the features of claim 1. The surface acoustic wave filter of the invention is defined by the features of claim 7. Thus, the invention described herein makes possible the advantages of providing an economical, efficient, reliable and compact package for hermetically packaging an electric device such as a quartz crystal resonator, a quartz filter, a quartz surface acoustic wave filter, and a hybrid IC that will not deteriorate due to age. These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a perspective view of a quartz crystal resonator according to Examples 1 to 3 of the present invention. Figure 2a is a cross-sectional view taken along the line a-a' of Figure 1 according to Example 1. Figure 2b is a cross-sectional view taken along the line b-b' of Figure 1 according to Example 1. Figure 2c is a plan view of a quartz plate according to Example 1. Figure 3 is a scheme showing a cleaned face of an ionic material according to the present invention. Figure 4a is a cross-sectional view taken along the line a-a' of Figure 1 according to Example 2. Figure 4b is a cross-sectional view taken along the line b-b' of Figure 1 according to Example 2. Figure 4c is a plan view of a quartz plate according to Example 2. Figure 5a is a cross-sectional view taken along the line a-a' of Figure 1 according to Example 3. Figure 5b is a cross-sectional view taken along the line b-b' of Figure 1 according to Example 3. Figure 5c is a plan view of a quartz plate according to Example 3. Figure 6 is a perspective view of a quartz crystal resonator according to Example 4 of the present invention. Figure 7a is a plan view of a quartz plate according to Example 4. Figure 7b is a cross-sectional view taken along the line a-a' of Figure 6 according to Example 4. Figure 7c is a cross-sectional view taken along the line b-b' of Figure 6 according to Example 4. Figure 8 is a perspective view of a quartz crystal resonator according to Example 5 of the present invention. Figure 9a is a cross-sectional view taken along the line a-a' of Figure 8 according to Example 5. Figure 9b is a cross-sectional view taken along the line b-b' of Figure 8 according to Example 5. Figure 9c is a plan view of a quartz plate according to Example 5. Figure 10 is a perspective view of a quartz filter according to Examples 6 to 8 of the present invention. Figure 11a is a cross-sectional view taken along the line a-a' of Figure 10 according to Example 6. Figure 11b is a cross-sectional view taken along the line b-b' of Figure 10 according to Example 6. Figure 11c is a plan view of a quartz plate according to Example 6. Figure 12a is a cross-sectional view taken along the line a-a' of Figure 10 according to Example 7. Figure 12b is a cross-sectional view taken along the line b-b' of Figure 10 according to Example 7. Figure 12c is a plan view of a quartz plate according to Example 7. Figure 13a is a cross-sectional view taken along the line a-a' of Figure 10 according to Example 8. Figure 13b is a cross-sectional view taken along the line b-b' of Figure 10 according to Example 8. Figure 13c is a plan view of a quartz plate according to Example 8. Figure 14 is a perspective view of a quartz filter according to Example 9 of the present invention. Figure 15a is a plan view of a quartz plate according to Example 9. Figure 15b is a cross-sectional view taken along the line a-a' of Figure 14 according to Example 9. Figure 15c is a cross-sectional view taken along the line b-b' of Figure 14 according to Example 9. Figure 16 is a perspective view of a quartz filter according to Example 10 of the present invention. Figure 17a is a cross-sectional view taken along the line a-a' of Figure 16 according to Example 10. Figure 17b is a cross-sectional view taken along the line b-b' of Figure 16 according to Example 10. Figure 17c is a plan view of a quartz plate according to Example 10. Figure 18 exemplarily shows another structure for electrodes according to Examples 6 to 10. Figure 19 shows a structure for a surface acoustic wave filter according to Example 11 of the present invention. Figure 20 shows a structure for a surface acoustic wave filter according to Example 12 of the present invention. Figure 21 shows a structure for a surface acoustic wave filter according to Example 13 of the present invention. Figure 22 shows a structure for a surface acoustic wave filter according to Example 14 of the present invention. Figure 23 shows a structure for a surface acoustic wave filter according to Example 15 of the present invention. Figure 24 shows an exemplary structure for a conventional quartz crystal resonator. Figure 25a is a front view showing an exemplary structure for a conventional quartz filter. Figure 25b is a back view showing the exemplary structure for the conventional quartz filter. Figure 26 shows an exemplary structure for a conventional surface acoustic wave filter. Figure 27 is a perspective view showing an exemplary structure for a conventional package. Figure 28 is a perspective view showing another quartz crystal resonator according to Example 1 of the present invention mounted on a surface of a circuit board. DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1 Hereinafter, the present invention will be described referring to the preferred examples using a quartz as a piezoelectric material for resonators and an ionic material for package members in some examples. A quartz crystal resonator according to Example 1 of the present invention will be described referring to Figures 1 and 2 . Figure 1 is a perspective view of the quartz crystal resonator according to Example 1. Figure 2a is a cross-sectional view taken along the line a-a' of Figure 1 , and Figure 2b is a cross-sectional view taken along the line b-b' of Figure 1 . Figure 2c is a top plan view of a quartz plate of the quartz crystal resonator. In Figure 1 , the quartz crystal resonator includes the quartz plate 101 , a covering quartz plate 102 , a base quartz plate 103 , and a through-hole 801 . The covering quartz plate 102 and the base quartz plate 103 are integrally and direct bonded with the quartz plate 101 at direct bonding faces 104 and 105 , respectively, without using adhesives. Herein, the direct bonding process will be described referring to Figure 3 . The direct bonding process is carried out after carefully and sufficiently cleaning the face of the ionic material. As shown in Figure 3 , a hydroxyl group (-OH) is formed on the face due to the presence of an electric charge of atoms. Furthermore, several layers of water molecules are formed around hydroxyl groups on the surface. When such two ionic materials each having a sufficiently flat face are attached to each other, they are firmly bonded together through direct bonding by the hydrogen bond formed among the hydroxyl groups and water molecules. Thus, the flat faces of the ionic materials can be direct bonded with each other without using adhesives. Herein, the direct bonding means the bonding utilizing adhesion generated between the ionic materials, which is achieved by cleaning the faces of the ionic materials such as quartz, aluminum oxide, and glass. Such bonding is called the direct bonding because no alien substances such as adhesives exist between the bonding faces. And a subsequent annealing makes the bonding more stronger. As for quartz, the annealing below 573°C at 0.101325 MPa (1 atmosphere) is desirable because of the stability of crystal structure. First, a structure for the quartz crystal resonator according to Example 1 will be described with reference to Figures 2a to 2c. The covering quartz plate 102 and the base quartz plate 103 each have a recessed portion at almost the central portion thereof. The electrodes 106 and 107 are disposed so as to be opposed to each other. Herein, the cutting angle and thickness of the quartz plate 101 and the shape, pattern, and thickness of the electrodes 106 and 107 are determined so as to obtain the desired resonance characteristics. In addition, through-holes 801 and 802 each filled with a conductive material such as metal, are formed in the covering quartz plate 102 and the base quartz plate 103. The electrode 106 is led to the outside via the bump 803 and the through hole 801 . The electrode 107 is led to the outside via the bump 804 and the through-hole 802 . The quartz plate 101 is integrally and direct bonded with the covering quartz plate 102 at the direct bonding face 104 , and with the base quartz plate 103 at the direct bonding face 105 . Next, an operation for the quartz crystal resonator of Example 1 will be discussed. Upon inputting an electrical signal to one of the electrodes 106 and 107 , the quartz plate 101 resonates at frequencies determined by the thickness and the elastic constant thereof. That is, the quartz crystal resonator functions as an electrical resonator having resonant frequencies determined by the thickness and the elastic constant of the quartz plate 101 . As is described above, the quartz plate 101 having the electrodes 106 and 107 on each face thereof has desired resonance characteristics. Without using adhesives, that is, by direct bonding such a quartz plate with two quartz plates constituting the package, it is possible to reduce the deterioration due to age. In addition, according to Example 1, the quartz plates having the same thermal expansion coefficient are bonded together, so that a compact and simple quartz crystal resonator excellent in thermal stability can be obtained. Besides, the electrodes can be led to the outside of the package while remaining the perfect packaging structure of the package by providing the through-holes 801 and 802 , and bumps 803 and 804 . Figure 34 shows another quartz crystal resonator 401 of the present example mounted on a surface of a circuit board 406 . A wiring pattern 403 is formed on the surface of the quartz crystal resonator 401 so as to electrically be connected to a through-hole 402 . A circuit pattern 404a is formed on the surface of the circuit board 406 . The wiring pattern 403 is connected to the circuit pattern 404a by using solder. Another through-hole (not shown) is formed on the bottom surface of the quartz crystal resonator 401 in a portion close to a circuit pattern 404b and connected to the circuit pattern 404b by using solder. Example 2 Hereinafter, a quartz crystal resonator according to Example 2 of the present invention will be described with reference to Figures 1 and 4 . Figure 1 is a perspective view of the quartz crystal resonator according to Example 2. Figure 4a is a cross-sectional view taken along the line a-a' of Figure 1 , and Figure 4b is a cross-sectional view taken along the line b-b' of Figure 1 . Figure 4c is a top plan view of the quartz crystal resonator. As shown in Figure 4 , the quartz crystal resonator of Example 2 has the same structure as that of Example 1 excepting that the quartz plate 101 has a recessed portion at one face thereof, and the base quartz plate 103 is made flat without having a recessed portion. An operation for the quartz crystal resonator of Example 2 is the same as that of Example 1, so that the description thereof will be omitted. Example 3 Hereinafter, a quartz crystal resonator according to Example 3 of the present invention will be described with reference to Figures 1 and 5 . Figure 1 is a perspective view of the quartz crystal resonator according to Example 3. Figure 5a is a cross-sectional view taken along the line a-a' of Figure 1 , and Figure 5b is a cross-sectional view taken along the line b-b' of Figure 1 . Figure 5c is a top plan view of the quartz crystal resonator. As shown in Figure 5 , the quartz crystal resonator of Example 3 has the same structure as that of Example 1 excepting that the quartz plate 101 has a recessed portion at each face thereof, and the covering quartz plate 102 and the base quartz plate 103 are both made flat without having a recessed portion. An operation for the quartz crystal resonator of Example 3 is the same as that of Example 1, so that the description thereof will be omitted. Example 4 Hereinafter, a quartz crystal resonator according to Example 4 of the present invention will be described with reference to Figures 6 and 7 . Figure 6 is a perspective view of the quartz crystal resonator according to Example 4. Figure 7a is a top plan view of the quartz crystal resonator. Figure 7b is a cross-sectional view taken along the line a-a' of Figure 6 , and Figure 7c is a cross-sectional view taken along the line b-b' of Figure 6 . As shown in Figure 7 , the quartz crystal resonator of Example 4 has the same structure as that of Example 1 excepting that the central portion of the quartz piece 101 is made thinner than the periphery thereof. An operation for the quartz crystal resonator of Example 4 is the same as that of Example 1, so that the description thereof will be omitted. As is described above, the quartz plate 101 having the electrodes 106 and 107 on each face thereof has desired resonance characteristics. By direct bonding such a quartz plate with two quartz plates constituting the package without using adhesives, it is possible to reduce the deterioration due to age. In addition, the quartz plates having the same thermal expansion coefficient are bonded together, so that a compact and simple quartz crystal resonator excellent in thermal stability can be obtained. In addition, the thickness of the area of the quartz plate 101 where the electrodes 106 and 107 are formed is made thinner than that of the areas where the quartz plate 101 is supported by the covering quartz plate 102 and the base quartz plate 103 , so that elastic vibrational energy at the quartz plate 101 can be trapped within the area where electrodes 106 and 107 are formed, thereby realizing a quartz crystal resonator having a high Q value. In order to obtain a high Q value, the thickness of periphery portion of the quartz plate 101 ( t2 ) should be (2n + 1)/2 times as thick as that of the central portion of the quartz plate 101 ( t1 ), where n is a positive integer such as 1, 2, 3 ... . The width of the periphery portion of the quartz plate 101 ( w2 ) is wider than that of the quartz plates 102 and 103 ( w1 ). More preferably, the width w2 is wider than the width w1 by about 100 µm or more. According to the above-mentioned structure, a deterioration of Q value and a change of resonance frequencies are both suppressed. In the present example, the thicknesses of the central portion and periphery portion of the quartz plate 101 are 130 µm and 195 µm, respectively, and the resonance frequency is 12.8 MHz. Example 5 Hereinafter, a quartz crystal resonator according to Example 5 of the present invention will be described with reference to Figures 8 and 9 . Figure 8 is a perspective view of the quartz crystal resonator according to Example 5. Figure 9a is a cross-sectional view taken along the line a-a' of Figure 8 , and Figure 9b is a cross-sectional view taken along the line b-b' of Figure 8 . Figure 9c is a plan view of the quartz plate 101. In Figure 8 , the quartz crystal resonator includes a quartz plate 101 , a covering quartz plate 102 , and a base quartz plate 103 . The covering quartz plate 102 and the base quartz plate 103 are integrally and direct bonded with the quartz plate 101 at direct bonding faces 104 and 105 , respectively, without using adhesives. Furthermore, through-holes 801 and 802 are formed in the covering quartz plate 102 and are filled with a conductive material. As shown in Figures 9a and 9b , electrodes 106 and 107 on the quartz plate 101 . The reference numerals 803 and 804 indicate bumps. The quartz crystal resonator of Example 5 has the same structure as that of Example 1 excepting that a through-hole 805 filled with a conductive material is formed in the quartz plate 101 . Herein, the cutting angle and thickness of the quartz plate 101 and the thickness and patterned shape of the electrodes 106 and 107 are determined so as to obtain desired resonance characteristics. In addition, the electrode 106 is electrically connected to the through-holes 801 and 802 via the bumps 803 and 804 , while the electrode 107 and the through-hole 805 are electrically connected to the through-hole 802 via the bump 804 . An operation for the quartz crystal resonator of Example 5 is the same as that of Example 1, so that the description thereof will be omitted. As is described above, the quartz plate 101 having the electrodes 106 and 107 on each face thereof has desired resonance characteristics. By direct bonding such a quartz plate with two quartz plates constituting the package without using adhesives, it is possible to reduce the deterioration due to age. In addition, according to Example 5, the quartz plates having the same thermal expansion coefficient are bonded together, so that a compact and simple quartz crystal resonator excellent in thermal stability can be obtained. In addition, the electrodes can be led to the outside of the package while the structure of the package remains perfect. In addition, by providing the through-hole 805 in the quartz plate 101 , external input/output terminals (herein, through-holes 801 and 802 ) can be collected on one face of the package, which makes it easier to mount the quartz crystal resonator on a circuit board. In Examples 1 to 5, each recessed portion of the quartz plate, the covering quartz plate, and the base quartz plate is formed by means of an ion-milling method by collision and sputtering of, say, inert gas ions such as Ar + , a sandblasting method by collision and sputtering of sand particles under the presence of air, or an etching method by chemical reaction with, say, hydrofluoric acid. In Examples 1 to 5, the ionic material for the covering member, the base member and the body member are not limited to quartz, but other ionic materials, for example fluorides such as LiF, NaF, and KF, oxides such as Al 2 O 3 , MgO, TiO 2 , and SiO 2 , chlorides such as LiCl, KCl, NaCl, and CdCl 2 , intermediate compounds such as MgAl 2 O 4 , solid solution, and amorphous materials such as glass can also be employed in the present invention. Furthermore, the ionic materials to be used in the present invention may be single-crystalline or polycrystalline materials. In the case where an amorphous material such as glass, or a polycrystalline material is used, the thermal expansion coefficient thereof does not show anisotropy but isotropy, generally. Therefore, the body member and the package member are not necessarily aligned in a specified crystal axis direction so as to avoid thermal stress. In Examples 1 to 5, each shape of every component constituting the package is not particularly limited. For example, each shape of the recessed portions and the through-holes is not limited to the above-mentioned shape, but it may be a circle, a triangle, or any other desired shape. In Examples 1 to 5, the material, thickness, and the patterned shape of the electrodes are not limited to the above as long as the desired resonance characteristics can be obtained. Example 6 Hereinafter, a quartz filter according to Example 6 of the present invention will be described with reference to Figures 10 and 11 . Figure 10 is a perspective view of the quartz filter according to Example 6. Figure 11a is a cross-sectional view taken along the line a-a' of Figure 10 , and Figure 11b is a cross-sectional view taken along the line b-b' of Figure 10 . Figure 11c is a top plan view of the quartz filter. In Figure 10 , the quartz filter includes a quartz plate 1201 , a covering quartz plate 1202 , a base quartz plate 1203 , and through-holes 1801 and 1802 . The covering quartz plate 1202 and the base quartz plate 1203 are integrally and direct bonded with the quartz plate 1201 at direct bonding faces 1204 and 1205 , respectively, without using adhesives. First, a structure for the quartz filter according to Example 6 will be described with reference to Figures 11a to 11c. The covering quartz plate 1202 and the base quartz plate 1203 each have a recessed portion at almost the central portion thereof. Electrodes 1206 and 1207 are disposed so as to be opposed to each other. Electrodes 1208 and 1209 are likewise disposed so as to be opposed to each other. Herein, the cutting angle and thickness of the quartz plate 1201 and the shape, pattern, and thickness of the electrodes 1206 to 1209 are determined so as to obtain the desired resonance characteristics. The quartz plate 1201 is direct bonded with the covering quartz plate 1202 at the direct bonding face 1204 , and with the base quartz plate 1203 at the direct bonding face 1205 . In addition, the through-holes 1801 to 1804 , each filled with a conductive material such as metal, are formed in the covering quartz plate 1202 and the base quartz plate 1203 . The electrodes 1206 to 1209 are led to the outside via the bumps 1805 to 1808 and the thorough holes 1801 to 1804 , respectively. Next, an operation for the quartz filter of Example 6 will be discussed. Generally, one of the electrodes 1206 and 1207 , and one of the electrodes 1208 and 1209 are earthed. When assuming that the electrodes 1207 and 1209 are earthed, and the electrodes 1206 and 1208 are used as an input terminal and an output terminal, respectively, the quartz plate 1201 resonates at the frequencies determined by the thickness and the elastic constant thereof, upon inputting an electrical signal to the electrode 1206. This resonance is mechanically propagated in the quartz plate 1201 , so that an electrical field is generated between the electrodes 1208 and 1209 , and consequently the electrical signal can be picked up from the electrode 1208. That is, the quartz filter can function as a band-pass filter for the frequencies which are determined by the thickness and the elastic constant of the quartz plate 1201. As is described above, the quartz plate 1201 having the electrodes 1206 and 1207 on each face thereof has the desired resonance characteristics. By direct bonding such a quartz plate with two quartz plates constituting the package without using adhesives, it is possible to reduce the deterioration due to age. In addition, according to Example 6, the quartz plates having the same thermal expansion coefficient are bonded together, so that a compact and simple quartz filter that is excellent in thermal stability can be obtained. Besides, the electrodes can be readily led to the outside of the package while the structure of the package remains perfect by providing the through-holes 1801 to 1804, and bumps 1805 to 1808. Example 7 Hereinafter, a quartz filter according to Example 7 of the present invention will be described with reference to Figures 10 and 12 . Figure 10 is a perspective view of the quartz filter according to Example 7. Figure 12a is a cross-sectional view taken along the line a-a' of Figure 10 , and Figure 12b is a cross-sectional view taken along the line b-b' of Figure 10 . Figure 12c is a top plan view of the quartz plate. As shown in Figures 12a to 12c , the quartz filter of Example 7 has the same structure as that of Example 6 excepting that the quartz plate 1201 has a recessed portion at one face thereof, and the base quartz plate 1203 is made flat without having a recessed portion. An operation for the quartz filter of Example 7 is the same as that of Example 6, so that the description thereof will be omitted. Example 8 Hereinafter, a quartz filter according to Example 8 of the present invention will be described with reference to Figures 10 and 13 . Figure 10 is a perspective view of the quartz filter according to Example 8. Figure 13a is a cross-sectional view taken along the line a-a' of Figure 10 , and Figure 13b is a cross-sectional view taken along the line b-b' of Figure 10 . Figure 13c is a top plan view of the quartz plate. As shown in Figures 13a to 13c , the quartz filter of Example 8 has the same structure as that of Example 6 excepting that the quartz plate 1201 has a recessed portion at each face thereof, and the covering quartz plate 1202 and the base quartz plate 1203 are both made flat without having a recessed portion. An operation for the quartz filter of Example 8 is the same as that of Example 6, so that the description thereof will be omitted. Example 9 Hereinafter, a quartz filter according to Example 9 of the present invention will be described with reference to Figures 14 and 15 . Figure 14 is a perspective view of the quartz filter according to Example 9. Figure 15a is a top plan view of the quartz plate. Figure 15b is a cross-sectional view taken along the line a-a' of Figure 14 , and Figure 15c is a cross-sectional view taken along the line b-b' of Figure 14 . In Figure 14 , the quartz filter includes a quartz plate 1201 , a covering quartz plate 1202 , and a base quartz plate 1203 . The quartz plate 1201 is integrally and direct bonded with the covering quartz plate 1202 and the base quartz plate 1203 at direct bonding faces 1204 and 1205 , respectively, without using adhesives. As shown in Figures 15a to 15c , electrodes 1206 to 1209 are formed on the quartz plate 1201 . The central portion of the quartz plate 1201 is made thinner than the periphery thereof, and the thickness and the cutting angle of the quartz plate 1201 and the thickness and the patterned shape of the electrodes 1206 to 1209 are determined so as to obtain the desired filter characteristics. An operation for thus obtained quartz filter of Example 9 is the same as that of Example 6, so that the description thereof will be omitted. As is described above, the quartz plate 1201 having the electrodes 1206 and 1207 on each face thereof has the desired resonance characteristics. By direct bonding such a quartz plate with two quartz plates constituting the package without using adhesives, it is possible to reduce the deterioration due to age. In addition, according to Example 9, the quartz plates having the same thermal expansion coefficient are bonded together, so that a compact and simple quartz filter that is excellent in thermal stability can be obtained. In addition, according to Example 9, the thickness of the area of the quartz plate 1201 where the electrodes 1206 to 1209 are formed is made thinner than that of the areas where the quartz plate 1201 is supported by the covering quartz plate 1202 and the base quartz plate 1203 , so that elastic vibrational energy at the quartz plate 1201 can be trapped within the area where electrodes 1206 and 1207 are formed, thereby realizing a quartz filter having a high Q value. The advantages provided by the above-mentioned structure of the quartz filter are the same as those of the quartz crystal resonator of Example 4. Example 10 Hereinafter, a quartz filter according to Example 10 of the present invention will be described with reference to Figures 16 and 17 . Figure 16 is a perspective view of the quartz filter according to Example 10. Figure 17a is a cross-sectional view taken along the line a-a' of Figure 16 , and Figure 17b is a cross-sectional view taken along the line b-b' of Figure 16 . Figure 17c is a plan view of the quartz plate 1201 . In Figure 16 , the quartz filter includes a quartz plate 1201 , a covering quartz plate 1202 , and a base quartz plate 1203 . The quartz plate 1201 is integrally and direct bonded with the covering quartz plate 1202 and the base quartz plate 1203 at direct bonding faces 1204 and 1205 , respectively, without using adhesives. Through-holes 1801 to 1804 each filled with a conductive material are formed in the covering quartz plate 1202 . As shown in Figures 17a to 17c , electrodes 1206 to 1209 are formed on the quartz plate 1201 . The reference numerals 1805 to 1808 indicate bumps. The quartz filter of Example 10 has the same structure as that of Example 6 excepting that through-holes 1809 and 1810 each filled with a conductive material are formed in the quartz plate 1201 . Herein, the cutting angle and thickness of the quartz plate 1201 and the patterned shape and thickness of the electrodes 1206 to 1209 are determined so as to obtain the desired resonator characteristics. In addition, the electrodes 1206 to 1209 are electrically connected to the through-holes 1801 to 1804 , 1809 , and 1810 via the bumps 1805 to 1808 . An operation for the quartz filter of Example 10 is the same as that of Example 6, so that the description thereof will be omitted. As is described above, the quartz plate 1201 having the electrodes 1206 and 1207 on each face thereof has the desired resonance characteristics. By direct bonding such a quartz plate with two quartz plates constituting the package without using adhesives, it is possible to reduce the deterioration due to age. In addition, according to Example 10, the quartz plates having the same thermal expansion coefficient are bonded together, so that a compact and simple quartz filter that is excellent in thermal stability can be obtained. Besides, the electrodes can be led to the outside of the package while the structure of the package remains perfect. In addition, by providing the through-holes 1809 and 1810 in the quartz plate 1201 , external input/output terminals (herein, through-holes 1801 to 1804 ) can finally be disposed on one face of the package, which makes it advantageous to mount the quartz filter on a circuit board. In Examples 6 to 10, each recessed portion of the quartz plate, the covering quartz plate, and the base quartz plate is formed by means of an ion-milling method by collision and sputtering of, say, inert gas ions such as Ar + , a sandblasting method by collision and sputtering of sand particles under the presence of air, or an etching method by chemical reaction with, say, hydrofluoric acid. In Examples 6 to 10, the ionic material for the covering member, the base member, and the body member are not limited to the quartz, but other ionic materials, for example fluorides such as LiF, NaF, and KF, oxides such as Al 2 O 3 , MgO, TiO 2 , and SiO 2 , chlorides such as LiCl, KCl, NaCl, and CdCl 2 , intermediate compounds such as MgAl 2 O 4 , solid solution, or amorphous materials such as glass can also be employed for the quartz plates in the present invention. Furthermore, the ionic materials to be used in the present invention may be single-crystalline or polycrystalline materials. In the case where an amorphous material such as glass, or a polycrystalline material is used, the thermal expansion coefficient thereof does not show anisotropy but isotropy, generally. Therefore the body member and the package member are not necessarily aligned in a specified crystal axis direction so as to avoid thermal stress. In Examples 6 to 10, each shape of every component constituting the package is not particularly limited. For example, each shape of the recessed portions and the through-holes is not limited to the above-mentioned shape, but it may be a circle, a triangle, or any other desired shape. In Examples 6 to 10, the materials, thickness, and the patterned shape of the electrodes are not limited to the above as long as the desired resonance characteristics can be obtained. In Examples 6 to 10, the number of electrodes is not limited to two pairs, but, for example, as shown in Figure 18 , three or more pairs of electrodes can be disposed on each face of a quartz plate 2201 (herein, electrodes 2202 , 2204 , and 2206 are disposed so as to be opposed to electrodes 2203 , 2205 , and 2207 , respectively). Example 11 Hereinafter, a surface acoustic wave filter according to Example 11 of the present invention will be described with reference to Figure 19 . In Figure 19 , the surface acoustic wave filter includes a quartz substrate 2301 and a covering member 2302 made of quartz. Herein, an electrode 2303 is disposed on the quartz substrate 2301 . Two through-holes 2304 each filled with a conductive material are formed in the quartz substrate 2301 as an input terminal and an output terminal, respectively. The electrode 2303 is electrically connected to the outside via the through-holes 2304 . At almost the central portion of the covering member 2302 , a recessed portion 2302 ' is formed so as to cover the electrode 2303 and the through-holes 2304 . Furthermore, the quartz substrate 2301 and the covering member 2302 are integrally and direct bonded with each other so that the electrode 2303 is packaged in the recessed portion 2302 '. Next, an operation for the surface acoustic wave filter of Example 11 will be described. As shown in Figure 19 , under the condition where the IDT type electrode 2303 is disposed on the face of the quartz substrate 2301 , when an electrical signal is inputted, the surface acoustic wave is excited on the face of the quartz substrate 2301 , and consequently only waves having certain frequencies can be propagated. Herein, the electrode pattern of the IDT type electrode 2303 and the effective piezoelectric constant of the quartz substrate 2301 determine which surface acoustic waves can be propagated. That is, such a surface acoustic wave filter can be used as a band-pass filter for passing only electrical signals with certain frequencies. As is described above, according to Example 11, the quartz substrate 2301 and the covering member 2302 are direct bonded with each other without using adhesives, which makes it possible to eliminate the additional space needed for the adhesives. More advantageously, no gas is caused in the hermetically packaged filter. Thus, a compact and high performable surface acoustic wave filter without causing deterioration due to age can be obtained. Example 12 Hereinafter, a surface acoustic wave filter according to Example 12 of the present invention will be described with reference to Figure 20 . In Figure 20 , the surface acoustic wave filter includes a quartz substrate 2401 , a covering member 2402 , and a base member 2403 . The covering member 2402 and the base member 2403 are both made of quartz. Herein, an electrode 2404 is disposed on the quartz substrate 2401 . Two bonding pads 2405 each filled with a conductive material are formed in the quartz substrate 2401 as an input terminal and an output terminal, respectively. The electrode 2404 is electrically connected to the outside via the bonding pads 2405 . At almost the central portion of the covering member 2402 , a recessed portion 2402 ' is formed so as to contain the quartz substrate 2401 . Furthermore, the quartz substrate 2401 and the base member 2403 are integrally and direct bonded with each other, while the base member 2403 and the covering member 2402 are integrally and direct bonded with each other. An operation for the surface acoustic wave filter of Example 12 is the same as that of Example 11, so that the description thereof will be omitted. As is described above, according to Example 12, the base member 2403 is direct bonded with the quartz substrate 2401 and the covering member 2402 without using adhesives, which makes it possible to eliminate the additional space needed for the adhesives. More advantageously, no gas is caused in the hermetically packaged. filter. Thus, a compact and high performable surface acoustic wave filter without causing deterioration due to age can be obtained. In addition, according to Example 12, the quartz substrate 2401 is mounted on the face of the base member 2403 where the electrode 2404 is not disposed. As a result, even if the circuit board on which the surface acoustic wave filter is mounted has a different thermal expansion coefficient from that of the quartz substrate 2401 , the thermal stress which is caused due to the difference in the thermal expansion coefficient can be absorbed, thereby advantageously reducing the change in the filter characteristics. Example 13 Hereinafter, a surface acoustic wave filter according to Example 13 of the present invention will be described with reference to Figure 21 . In Figure 21 , the surface acoustic wave filter includes a quartz substrate 2501 , a covering member 2502 , and a base member 2503 . The covering member 2502 and the base member 2503 are both made of quartz. Herein, an electrode 2504 is disposed on the quartz substrate 2501 . Two through-holes 2505 each filled with a conductive material are formed in the quartz substrate 2501 as an input terminal and an output terminal, respectively. The electrode 2504 is electrically connected to the outside via the through-holes 2505 . At almost the central portion of the covering member 2502 , a recessed portion 2502 ' is formed so as to cover the quartz substrate 2501 . Furthermore, the quartz substrate 2501 and the covering member 2502 are integrally and direct bonded with each other. An operation for the surface acoustic wave filter of Example 13 is the same as that of Example 11, so that the description thereof will be omitted. As is described above, according to Example 13, the quartz substrate 2501 and the covering member 2502 are direct bonded with each other without using adhesives, which makes it possible to eliminate the additional space needed for the adhesives. More advantageously, no gas is caused in the hermetically packaged filter. Thus, a compact and high performable surface acoustic wave filter without causing deterioration due to age can be obtained. In addition, according to Example 13, the quartz substrate 2501 is mounted on the face of the base member 2503 where the electrode 2504 is not disposed. As a result, even if the circuit board on which the surface acoustic wave filter is mounted has a different thermal expansion coefficient from that of the quartz substrate 2501 , the thermal stress which is caused due to the difference in the thermal expansion coefficient can be absorbed, thereby advantageously reducing the change in the filter characteristics. Herein, it is not necessary to direct bond the quartz substrate 2501 and the base member 2503 with each other since the electrode 204 has already been packaged by the quartz substrate 2501 and the recessed portion 2502 '. Example 14 Hereinafter, a surface acoustic wave filter according to Example 14 of the present invention will be described with reference to Figure 22 . In Figure 22 , the surface acoustic wave filter includes a quartz substrate 2601 and a covering member 2602 . An electrode 2603 is disposed on the quartz substrate 2601 . Two through-holes 2604 each filled with a conductive material are formed in the quartz substrate 2601 as an input terminal and an output terminal, respectively. The electrode 2603 is electrically connected to the outside via the through-holes 2604 . At almost the central portion of the covering member 2602 , a recessed portion 2602 ' is formed so as to cover the quartz substrate 2601 . Furthermore, the area where the quartz substrate 2601 and the covering member 2602 are direct bonded with each other is previously covered with an ionic material, thus forming an ionic material covering portion 2605 . In Example 14, the quartz substrate 2601 and the covering member 2602 are direct bonded with each other using adhesive force generated between the ionic materials thereof. An operation for the surface acoustic wave filter of Example 14 is the same as that of Example 11, so that the description thereof will be omitted. As is described above, according to Example 14, the quartz substrate 2601 and the covering member 2602 are direct bonded with each other via the ionic material covering portion 2605 without using adhesives, which makes it possible to eliminate the additional space needed for the adhesives. More advantageously, no gas is caused in the hermetically packaged filter. Thus, a compact and high performable surface acoustic wave filter without causing deterioration due to age can be obtained. Furthermore, by providing the ionic material covering portion 2605 , the quartz substrate 2601 and the covering member 2602 can be made considering the filter characteristics (elastic constant, electrical and mechanical coupling constant, etc.), workability, and production cost so as to optimize the surface acoustic wave filter. As a method for covering the ionic material covering portion 2605 with the ionic material, a chemical vapor deposition (CVD) method in which the raw material for the ionic material is decomposed in a vapor condition so as to deposit ionic material, a sputtering method in which a target, which is the raw material for the ionic material in vacuum comes into collision with high speed charged particles so as to be vaporized, and the like may be employed. In the case where an oxide of the piezoelectric material (namely, in this example, quartz substrate 2601 ) and the covering member 2602 is ionic, a method in which the face of a parent material is oxidized so as to form a thin oxide layer can be employed. In the case where the CVD method or sputtering method is employed, a buffer layer can be formed so as to achieve excellent bonding between the ionic material to be formed at a face and the parent material, thereby stably forming the ionic material on the face of the parent material. Example 15 Hereinafter, a surface acoustic wave filter according to Example 15 of the present invention will be described with reference to Figure 23 . In Figure 23 , the surface acoustic wave filter includes a quartz substrate 2701 , a covering member 2702 , and a base member 2703 . The covering member 2702 and the base member 2703 are both made of quartz. An electrode 2704 is disposed on the quartz substrate 2701 . Two bonding pads 2705 each filled with a conductive material are formed in the quartz substrate 2701 as an input terminal and an output terminal, respectively. The electrode 2704 is electrically connected to the outside via the bonding pads 2705 . At almost the central portion of the base member 2703 , a recessed portion 2703 ' is formed so as to contain the quartz substrate 2701 . Furthermore, the base member 2703 is integrally and direct bonded with the quartz substrate 2701 and the covering member 2702 . An operation for the surface acoustic wave filter of Example 15 is the same as that of Example 11, so that the description thereof will be omitted. As is described above, according to Example 15, the base member 2703 is direct bonded with the quartz substrate 2701 and the covering member 2702 without using adhesives, which makes it possible to eliminate the additional space needed for the adhesives. More advantageously, no gas is caused in the hermetically packaged filter. Thus, a compact and high performable surface acoustic wave filter without causing deterioration due to age can be obtained. In addition, according to Example 15, the quartz substrate 2701 is mounted on the face of the base member 2703 where the electrode 2704 is not disposed. As a result, even if a circuit board on which the surface acoustic wave filter is mounted has a different thermal expansion coefficient from that of the quartz substrate 2701 , the thermal stress which is generated due to the difference in the thermal expansion coefficient can be absorbed, thereby advantageously reducing the change in the filter characteristics. In Examples 11 to 13, and 15, the covering member and the base member are made of quartz, but they may be made of any other materials as long as the direct bonding can be achieved. In Examples 11 to 15, the piezoelectric substrate is made of quartz, but it may be made of any other materials such as lithium niobate (LiNbO 3 ) and lithium tantalate (LiTaO 3 ). Even if the piezoelectric substrate is made of materials other than the quartz, the same effects can be obtained as long as the piezoelectric substrate is direct bonded with the covering member and the base member. In Examples 11 to 15, the electrodes are led to the outside via the through-holes or bonding pads. However, the method for leading the electrodes to the outside is not limited to the above method. In the case where earthed electrodes are used, the method for leading the electrodes to the outside is not limited to the above method. In Examples 13 and 15, the covering member and the base member can be made of the same material, so that they can have the same thermal expansion coefficient, thereby realizing a higher reliable surface acoustic wave filter. In Examples 11 to 15, even if the piezoelectric substrate is made of a material other than the quartz, such as lithium niobate (LiNbO 3 ) and lithium tantalate (LiTaO 3 ), the covering member and the base member can be made of the same material as that of the piezoelectric substrate to have the same thermal expansion coefficient, thereby realizing a high reliable surface acoustic wave filter. In Examples 11 to 13, and 15, the covering member and the base member can be covered with the ionic material in the same manner as the covering member of Example 14. As a result, the covering member and the base member can be made considering the workability, and production cost so as to provide a higher performable surface acoustic wave filter at a low cost. Even when a material other than the quartz, such as lithium niobate (LiNbO 3 ) and lithium tantalate (LiTaO 3 ) is used, the same effects can be obtained. In Examples 11 to 15, the materials for the covering member and the base member are not limited particularly, but any other ionic materials, for example, fluorides such as LiF, NaF, and KF, chlorides such as LiCl, KCl, NaCl, and CdCl 2 , oxides such as Al 2 O 3 , MgO, SiO 2 , and TiO 2 , intermediate compounds such as MgAl 2 O 4 , solid solution, and amorphous materials such as glass can also be employed in the present invention. Furthermore, the ionic materials to be used in the present invention may be single-crystalline or polycrystalline materials. In the case where an amorphous material such as glass, or a polycrystalline material is used, the thermal expansion coefficient thereof does not show anisotropy but isotropy. Therefore, the body member and the covering member are not necessary aligned in a specified crystal axis direction so as to avoid thermal stress. In addition, by appropriately choosing the compositions of the ionic material to be used, it is possible to make the piezoelectric substrate, the covering member, and the base member have the same thermal expansion coefficient, thereby realizing a thermally reliable package. As an ionic material used in the device of the present invention, an ionic material which has covalent bonding property can be used as long as the direct bonding can be formed. In general, an ionic material has ionic bonding property as well as covalent bonding property as described in, for example, "Introduction to Solid State Physics", 5th edit., by Charles Kittel (1976), John Wiley & Sons Inc. In Examples 11 to 15, the IDT type electrodes alone are shown, but other electrodes such as earthed electrodes and reflecting electrodes may additionally be provided. Moreover, the electrode pattern of the electrodes on the piezoelectric substrate is not particularly limited. According to the present invention, the package can be obtained through direct bonding. As a result, the package of the present invention does not require adhesives, thereby not causing gas, and eliminating the additional space needed for the adhesives. Consequently, a compact, high performable, and economical quartz crystal device or package capable of performing excellent hermetic packaging operation without causing deterioration due to age can be obtained. In addition, according to the present invention, the covering member, the base member, and the body member constituting the package can be made of the same materials, so that each of them has the same thermal expansion coefficient, thereby realizing a quartz crystal device and a package that is excellent in thermal stability. According to the present invention, by means of an ion-milling method, a sandblasting method, or a chemical etching method using chemical reaction by hydrofluoric acid or the like, the quartz plate having the desired resonance characteristics or filter characteristics can be thinly polished, thereby realizing a quartz device having an extremely high Q value. According to the present invention, the electrode on one face of the quartz plate can be led to the opposite face via the through-hole filled with a conductive material, so that the package can be more readily mounted on a circuit board. According to the present invention, the through-hole filled with the conductive material can be formed in the supported quartz plate, and a conductive bump can be formed so as to extend from the through-hole. Thus, the conductive bump can be pressed onto an electrode portion of the quartz plate where the electrode is formed so as to lead the electrode to the outside, thereby realizing a quartz filter capable of performing an excellent hermetic packaging. In the case where a parent material is coated with an ionic material, the material for the parent material can arbitrarily be determined considering the processability and the production cost, etc. Therefore, the package can readily be realized with a desired shape, at a low cost. Various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope of the claims.

Claims: A piezoelectric device comprising: a piezoelectric plate (101; 1201) provided with at least one pair of electrodes (106, 107; 1206, 1207) opposed to each other at almost a central portion of each major surface (104, 105; 1204, 1205) of the piezoelectric plate (101; 1201); a first plate (102; 1202) made of an ionic material to cover at least one electrode (106, 107; 1206, 1207) of said pair of electrodes at one major surface of the piezoelectric plate (101; 1201) such that said first plate (102; 1202) faces said piezoelectric plate (101; 1201), thereby forming a recess; a second plate (103; 1203) made of an ionic material to cover at least the other electrode (106, 107; 1206, 1207) of said pair of electrodes at another major surface to face said piezoelectric plate (101; 1201) thereby forming a recess; wherein said first plate (102; 1202) and said second plate (103; 1203) are directly bonded with said piezoelectric plate (101; 1201) so that said piezoelectric plate (101; 1201) is sandwiched therebetween thereby packaging each of said at least one pair of electrodes (106, 107; 1206, 1207). A piezoelectric device according to claim 1, wherein the first plate (102; 1202) and the second plate (103; 1203) each have a recessed portion to cover said at least one electrode (106, 107; 1206, 1207) at one major surface to face said piezoelectric plate (101; 1201). A piezoelectric device according to claim 1 or 2, wherein said piezoelectric plate (101; 1201), said first plate (102; 1202) and said second plate (103; 1203) are all made of quartz. A piezoelectric device according to one of the preceding claims, wherein said at least one electrode (106, 107; 1206, 1207) on one major surface of said piezoelectric plate (101; 1201) is led to the other major face of said piezoelectric plate (101; 1201) via at least through-hole (801; 1801) filled with a conductive material. A piezoelectric device according to one of claims 1, 3 and 4, wherein the piezoelectric plate (101; 1201) is provided with at least one electrode (106, 107; 1206, 1207) at almost a central portion corresponding to a thinner portion of each major surface, one major surface thereof has a recessed portion, the other major surface thereof is made flat; the first plate (102; 1202) has a recessed portion to cover said at least one electrode (106, 107; 1206, 1207) at one major surface to face said flat major surface of said piezoelectric plate (101; 1201); and the second plate has a flat major surface to face said major surface having said recessed portion of said piezoelectric plate (101; 1201). A piezoelectric device according to one of claims 1, 3 and 4, wherein the piezoelectric plate (101; 1201) is provided with at least one electrode (106, 107; 1206, 1207) at almost a central portion corresponding to a thinner portion of each major surface having a recessed portion, the first plate (102; 1202) and the second plate (103; 1203) each have a flat major surface to face said piezoelectric plate (101; 1201). A surface acoustic wave filter comprising: a piezoelectric substrate (2301, 2401, 2501, 2601, 2701) provided with a plurality of electrodes (2303, 2404, 2504, 2603, 2704) at one major surface, and a covering member (2302, 2402, 2502, 2602, 2702) made of an ionic material at a major surface and having a recessed portion to cover said plurality of electrodes (2303, 2404, 2504, 2603, 2704) at one major surface to face said piezoelectric substrate (2301, 2401, 2501, 2601, 2701); wherein said piezoelectric substrate (2301, 2401, 2501, 2601, 2701) is directly bonded with said covering member (2302, 2402, 2502, 2602, 2702) thereby packaging said plurality of electrodes (2303, 2404, 2504, 2603, 2704). A surface acoustic wave filter according to claim 7, wherein said piezoelectric substrate (2301, 2401, 2501, 2601, 2701) is made of quartz, lithium niobate LiNbO 3 or lithium tantalate LiTaO 3 . A surface acoustic wave filter according to claim 7 or 8, wherein said piezoelectric substrate (2301, 2401, 2501, 2601, 2701) and said covering member (2302, 2402, 2502, 2602, 2702) are made of the same material. A surface acoustic wave filter according to one of claims 7 to 9, wherein a stress buffering plate is attached to a major surface opposite to said major surface of said piezoelectric substrate (2301, 2401, 2501, 2601, 2701) where said plurality of electrodes (2303, 2404, 2504, 2603, 2704) are formed. A surface acoustic wave filter according to one of claims 7 to 10, wherein a plurality of through-holes (2304, 2505, 2604) filled with conductive material are formed in said piezoelectric substrate (2301, 2501, 2601) so as to electrically connect said plurality of electrodes (2303, 2504, 2603) to the outside.

Inventors: HASHIMOTO KOJI, ISHIZAKI TOSHIO, SAKAUE TSUYOSHI, SATOH YUKI, UWANO TOMOKI, YAMADA TOHRU, HASHIMOTO, KOJI, ISHIZAKI, TOSHIO, SAKAUE, TSUYOSHI, SATOH, YUKI, UWANO, TOMOKI, YAMADA, TOHRU
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1994
Main classification: H01L41
IPC codes: H03H9, H01L41